PHY007 · Mechanics · Classes 9–12
Conservation of momentum
Objective: To verify the conservation of linear momentum in a collision between two trolleys that stick together.
🧰 Apparatus
- Low-friction track
- Two trolleys with Velcro pads
- Light gates or ticker timer
- Balance
📐 Formulas and SI units
p = mv (kg m/s)
m₁u₁ + m₂u₂ = (m₁ + m₂)v
💡 Theory
When no external force acts, the total momentum of a system stays constant. In a perfectly inelastic collision the trolleys move off together: m₁u₁ + m₂u₂ = (m₁ + m₂)v. Kinetic energy is not conserved; some becomes heat and sound.
🪜 Procedure
- Measure both masses.
- Push trolley 1 towards stationary trolley 2.
- Measure the speed before the collision and the common speed after.
- Calculate total momentum before and after.
- Repeat with different masses.
📋 Observations Simulated values
✅ Result and conclusion
A 2 kg trolley at 3 m/s hitting 1 kg at rest moves off at 2 m/s: momentum 6 kg m/s before and after; 3 J of kinetic energy is lost.
🔬 In a real lab
Track friction and air drag remove a little momentum; tilt the track slightly to compensate for friction.
⚠️ Safety
Place end stops on the track so trolleys cannot fall off.
❓ Quick check
1. Momentum is conserved when…
2. In a sticky collision, kinetic energy is…
3. Unit of momentum?
🏋️ Practice
Easy1 kg at 4 m/s hits 1 kg at rest; they stick. v?
Answer: 2 m/s
MediumGun recoil: 0.02 kg bullet at 400 m/s from a 4 kg gun. Gun speed?
Answer: 2 m/s
ChallengeTwo 1 kg carts at 2 m/s head-on stick. v?
Answer: 0